Windows including glass substrates with solar control coatings provided thereon are known in the art. Such windows may be used in the context of architectural windows, automotive windows, and/or the like.
Commercial architectural applications often require that certain windows in a building be heat treated (HT) (e.g., thermally tempered) while others need not be. Given that some buildings require the use of both heat treated (HT) and non-HT windows, it will be appreciated by those skilled in the art that there sometimes exists a need in the art to approximately color match HT and non-HT windows in the same building. Color matching is often thought of in terms of a normal viewing angle (i.e., directly on-axis or 0 degree viewing angle looking straight at the window).
However, color at high viewing angles may also be important in certain applications. For example, significant coloration (e.g., very red or very yellow) at high viewing angles (e.g., 45 and/or 60 degrees off-axis) are often undesirable.
WO 02/04375 (corresponding to U.S. Ser. No. 09/794,224, hereby incorporated herein by reference) discloses a heat treated (HT) low-E coating including a pair of silver layers and numerous dielectric layers. The Examples 1–3 in WO 02/04375 are primarily designed for automotive uses such as vehicle windshields. Accordingly, the Examples of WO 02/04375 utilize silvers layers that are about 105 Å thick in an effort to achieve high visible transmission. This desire for very high visible transmission and automotive characteristics results in a rather high TS % (Total Solar energy transmittance) of about 46.75% (see Table 8 on page 38) and a rather high SHGC (Solar Heat Gain Coefficient) well above 0.4.
However, in architectural applications (e.g., for use in IG window units), TS % and SHGC are more important and are generally desired to be lower than the values realized by the Examples in WO 02/04375. Thus, when the coating of WO 02/04375 was tailored for architectural applications, the result was a thicker upper silver layer (130 Å) in order to lower the TS % and SHGC to desirable ranges. A resulting commercially sold HT architectural coating in this regard is set forth below (this was used in IG units), with the layers listed in order form the glass substrate outwardly.
Conventional HT Coated Article
LayerThicknessGlass Substrate1–10mmTiO275ÅSi3N4165ÅNiCrOx26ÅAg98ÅNiCrOx26ÅSnO2525ÅSi3N4165ÅNiCrOx26ÅAg130ÅNiCrOx26ÅSnO2100ÅSi3N4210Å
The aforesaid conventional HT coated article, has the following color characteristics at both a normal viewing angle (i.e., straight-on 0 degree viewing angle) and higher viewing angles (VA) such as 45 degrees and 60 degrees off-axis (measured monolithically).
TABLE 1Color Shift Characteristics of Article from AboveColor Characteristic0 deg. VA45 deg. VA60 deg. VAGlass Side Reflectivea*−0.310.11−0.35b*−0.733.464.66L*26.9731.6545.29Δb*gn/a4.195.39Film Side Reflectivea*−1.350.980.79b*3.446.516.16L*25.5730.9445.61
It can be seen from the above that while the glass side a* color value is fairly stable and neutral upon viewing angle (VA) change from 0 to 60 degrees, the glass side b* color value realizes significant change/shift upon viewing angle change from 0 to 45 and/or 60 degrees (i.e., Δb* is high). More importantly, the b* value moves drastically toward and into a high positive b* region (e.g., b*=4.66 at the 60 degree viewing angle). These high b* values at the high viewing angles of 45 and 60 degrees are undesirable since they cause the coated article to appear yellow to a viewer when viewed at such angles. This yellow coloration (i.e., high b* value) at high viewing angles is problematic in that: (a) it can be aesthetically displeasing to the eye, and/or (b) it can prevent a HT coated article such as the one described above from colorwise matching a non-HT counterpart having fairly neutral off-axis coloration.
The undesirable glass side reflective b* color shift of the aforesaid coating is illustrated as the “conventional” coating in FIG. 3 (i.e., the line with the boxes). FIG. 3 clearly illustrates the undesirable high b* color values realized by the aforesaid conventional coating at viewing angles of 45 and 60 degrees.
Accordingly, it will be appreciated by those skilled in the art that there exists a need in the art for a coating which realizes a more neutral (i.e., closer to zero) b* value at high viewing angles, and which is still capable of achieving low SHGC and/or TS % values.